Hierarchical Storage Management (HSM) includes a primary storage medium (for example, HDD (Hard Disk Drive)) with a high access speed and a low-cost secondary storage medium (for example, tape medium) with a low access speed, and a file with a low frequency of use is automatically moved from the primary storage medium with a high access speed to the secondary storage medium with a low access speed.
When a sequential-access storage medium, such as a tape, is used as the secondary storage medium, a file moved from the primary storage medium is additionally written as a new file on the secondary storage medium.
Furthermore, when a file recorded on the secondary storage medium is updated on the primary storage medium, the file previously recorded on the secondary storage medium becomes old and is handled as an invalid data.
In this case, the file on the secondary storage medium is not deleted actually, and the file is just handled as an invalid file on a register that manages files.
Therefore, with an increase of invalid files, recording areas of valid files on the secondary storage medium are fragmented, and the files are discontinuously recorded in a plurality of areas (FIG. 1).
Therefore, to eliminate the fragmentation of the recording areas of the valid files in the HSM that uses a tape medium as the sequential-access secondary storage medium, there is a method of repacking and additionally writing, in another secondary storage medium, the valid files on the secondary storage medium with a degree of fragmentation equal to or greater than a certain threshold (FIG. 2).
This method is called reclamation.
This operation is not related to file operation performed by a user and is a process executed on the background.
However, a total of two tape drives for reading files from a source medium and for writing files in a destination medium are occupied for the execution.
Therefore, it is preferable that the state without the occurrence of the process continues.
Furthermore, when the secondary storage medium is a tape medium, a head of the tape drive moves to a writing or reading position of a file upon access to the file on the secondary storage medium (FIGS. 3 and 4).
Particularly, a sequential-access storage medium, such as a tape, has a characteristic that the access time to a valid file is long when valid files and invalid files are mixed on the medium.
This is because the movement distance of the head of the tape drive to access the target file becomes long due to the existence of the invalid files. Since a process of file operation, such as writing and reading of a file, is not executed during the movement of the head, much time is spent in using the tape drive for a process of just moving the head.
Patent Literature 1 and Patent Literature 2 describe techniques for efficiently performing elimination (reclamation) of fragmentation of files regarding a tape medium.
Patent Literature 3 describes a technique of reusing free areas.
However, none of Patent Literatures 1 to 3 describe a characteristic configuration of the present invention described later.